Waste plastic recycling method

Abstract

The invention belongs to the technical field of plastic recovery, and particularly relates to a waste plastic recovery treatment method which comprises the following steps: S1, pretreatment: sorting, crushing and cleaning waste plastics; s2, low-temperature critical extraction: feeding the pretreated plastic into a high-pressure reaction kettle, injecting an extraction solvent into the high-pressure reaction kettle, extracting under the conditions that the temperature is 50-100 DEG C lower than the melting point of the plastic and the pressure is higher than the critical pressure of the extraction solvent, and separating to remove additives and small molecular impurities; s3, in-situ reconstruction: after extraction, adding a reconstruction agent into the high-pressure reaction kettle, and carrying out a molecular chain repair reaction in a supercritical fluid atmosphere; and S4, the reconstructed plastic melt is extruded and granulated. Extraction is carried out at the temperature lower than the melting point of plastic by 50-100 DEG C, a plastic polymer main body is kept in a solid state and is not molten, a thermal oxygen chain scission reaction is avoided, and a molecular chain structure is reserved; and reconnection and growth of the molecular chain are realized through reconstruction.

Description

Technical Field

[0001] This invention belongs to the field of plastic recycling technology, and in particular to a method for recycling and processing waste plastics. Background Technology

[0002] With the continued growth in the consumption of plastic products, the treatment and recycling of waste plastics has become a global environmental challenge. Currently, waste plastic recycling mainly faces two major technological bottlenecks: First, there is the performance degradation problem caused by thermo-oxidative aging. Traditional mechanical recycling processes typically employ high-temperature melting and reprocessing (temperature > 200℃). During this process, polymer molecular chains undergo chain-splitting reactions under high-temperature shear and in the presence of oxygen, resulting in a significant decrease in molecular weight. Taking polyethylene terephthalate (PET) as an example, the intrinsic viscosity after recycling typically decreases from 0.75-0.85 dL / g of the virgin material to below 0.55-0.65 dL / g. With each recycling cycle, the mechanical properties of the material deteriorate, rendering it only suitable for low-value-added products.

[0003] Secondly, there is the problem of performance control issues caused by the mixture of complex additives. Waste plastic raw materials typically contain a variety of additives, including plasticizers, flame retardants (such as polybrominated diphenyl ethers and hexabromocyclododecane), color masterbatches, light stabilizers, and heat stabilizers. In traditional recycling processes, these additives cannot be effectively separated. They react with each other, decompose, and carbonize in a molten state at high temperatures, leading not only to irritating odors and the release of small molecules as pollution from recycled materials, but also to large and uncontrollable fluctuations in the performance of recycled materials. In particular, waste household appliance plastics containing bromine-based flame retardants often produce toxic and corrosive gases after recycling due to the decomposition of the flame retardants, severely restricting their high-value utilization.

[0004] Existing chemical recycling technologies (such as alcoholysis, hydrolysis, and pyrolysis) can depolymerize polymers into monomers or oligomers, but they require extremely high purity of the feedstock. Complex additives in waste plastics can poison catalysts and contaminate depolymerization products, and the processes are lengthy, energy-intensive, and economically inefficient. While pyrolysis technology can process low-residual-value mixed waste plastics, the product is a mixture of oils rather than high-value polymer materials. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for recycling and processing waste plastics, so as to solve the above-mentioned problem.

[0006] To solve the above problems, the technical solution adopted by the present invention is: a method for recycling and processing waste plastics, comprising the following steps: S1. Pre-treatment: sorting, crushing and washing waste plastics; S2. Low-temperature critical extraction: The pretreated plastic is sent into a high-pressure reactor and an extraction solvent is injected into the high-pressure reactor. Extraction is carried out at a temperature 50-100°C lower than the melting point of the plastic and at a pressure higher than the critical pressure of the extraction solvent to separate and remove additives and small molecule impurities. S3, In-situ Reconstruction: After extraction, a reconstruction agent is added to the high-pressure reactor, and a molecular chain repair reaction is carried out in a supercritical fluid atmosphere; S4. Extrude and granulate the reconstructed plastic melt.

[0007] Further, in step S2, the extraction solvent includes a main solvent and a co-solvent. The main solvent is carbon dioxide, propane or dimethyl ether, and the co-solvent is ethanol, acetone or ethyl acetate. The main solvent is first pressurized and liquefied, and then the main solvent and co-solvent are introduced into a mixer. After mixing, the mixture is introduced into a high-pressure reactor. In the mixer, the volume ratio of the main solvent is 60-90%.

[0008] Further, in step S2, the extraction pressure is 8-25 MPa, the extraction time is 30-120 min, the rate at which the extraction solvent is introduced into the high-pressure reactor is 0.5-5 L / min, and the weight ratio of the extraction solvent to the plastic is (3-10):1.

[0009] Furthermore, in step S2, the extraction solvent discharged from the high-pressure reactor is fed into a distillation column to separate the additives and small molecule impurities in the extraction phase, thereby obtaining an extraction solvent that can be reused.

[0010] Further, in step S3, the reconstructing agent is a four-armed star copolymer having a triazine-1,3,5-tribenzamide supramolecular core, a polycaprolactone flexible linker, and glycidyl methacrylate-butyl methacrylate copolymer side chains, with the following general structural formula: ,

[0011] Among them, ST is the supramolecular unit of triazine-1,3,5-tribenzamide, PCL is polycaprolactone with a molecular weight of 2000-5000, GMA-co-BMA is the side chain of a random copolymer of glycidyl methacrylate and butyl methacrylate with a degree of polymerization m=10-30, and the molar ratio of GMA to BMA is 3:1 to 1:1.

[0012] Furthermore, the weight-average molecular weight of the reconstructing agent is 3.0 × 10⁻⁶. 4 -6.0×10 4 The molecular weight distribution is ≤1.5, and the epoxy equivalent is 350-450 g / eq.

[0013] Furthermore, the preparation method of the reconstructing agent is as follows: Synthesis of the supramolecular core ST of triazine-1,3,5-tribenzamide; ST-PCL-ST prepolymer was synthesized by esterification reaction of ST with dihydroxyl-terminated polycaprolactone. ST-PCL-ST was reacted with 2-bromoisobutyryl bromide to introduce ATRP initiation sites, thus obtaining a macromolecular initiator; A reconstructing agent was obtained by grafting the side chains of glycidyl methacrylate and butyl methacrylate copolymers onto a macromolecular initiator via atom transfer radical polymerization.

[0014] Furthermore, step S3 is carried out in a carbon dioxide atmosphere, specifically including: S31. Add the reconstructing agent to the high-pressure reactor. The amount of reconstructing agent added is 1.0-5.0% of the plastic mass. S32. Mix at 150-180℃ for 5-10 minutes to ensure uniform dispersion of the reconstructing agent; S33. Cool to 80-100℃ and hold for 10-20 minutes to promote supramolecular self-assembly; S34. Heat to 200-240℃ and react for 15-30 minutes to achieve covalent chain extension.

[0015] Further, in step S1, during cleaning, the broken plastic is immersed in a cleaning solution containing a cationic surfactant and cleaned at 60-75°C for 20-40 minutes, controlling the residual amount of cationic surfactant on the plastic surface to be 0.01-0.1 wt% after cleaning; the cationic surfactant is hexadecyltrimethylammonium bromide or a quaternary ammonium salt surfactant with C12-C18 alkyl chains. In step S2, the process is carried out under a supercritical carbon dioxide atmosphere. The cationic surfactants remaining on the plastic surface form reverse micelles, which encapsulate the extracted additives and prevent them from redepositing on the plastic surface. In step S3, the quaternary ammonium cations of the cationic surfactant remaining on the plastic surface form a cation-π interaction with the triazine ring of the ST unit. At the same time, the C16 long chain of the cationic surfactant becomes entangled with the PCL chain segment, which together promotes the orientation self-assembly of the reconstructing agent.

[0016] Furthermore, the cleaning solution comprises, by weight, 0.5-2.0 parts of hexadecyltrimethylammonium bromide, 0.3-1.5 parts of nonionic surfactant, 1.0-3.0 parts of bio-based solvent, 0.1-0.3 parts of complexing agent, and 93.2-98.1 parts of water.

[0017] The beneficial effects of this invention are: the extraction is carried out at a temperature 50-100°C below the melting point of plastics. During the extraction stage, the main body of the plastic polymer remains solid and does not melt, which fundamentally avoids the thermo-oxidative chain breaking reaction caused by traditional high-temperature melting and recycling (>200°C). The molecular chain structure is preserved, providing a high-quality molecular chain basis for subsequent reconstruction.

[0018] Under conditions exceeding the critical pressure of the extraction solvent, the solvent operates in a subcritical or supercritical state, possessing both the high permeability of gases and the high solubility of liquids. Solvent molecules can penetrate into the polymer matrix, causing it to swell. Plasticizers, flame retardants, small molecules of color masterbatches, unreacted monomers, and odor molecules are selectively dissolved due to differences in solubility parameters and carried out of the matrix along with the solvent molecules, resulting in a high removal rate. This method solves the problems of strong odor, small molecule precipitation, and unstable performance of recycled materials in traditional recycling processes in one step.

[0019] Following extraction, a remodeling agent is added to the same reaction system under a supercritical fluid atmosphere. The supercritical fluid, acting as the reaction medium, promotes the uniform dispersion and penetration of the remodeling agent into the polymer matrix, enabling it to fully contact and chemically react with the active end groups (such as hydroxyl, carboxyl, and double bonds) generated by chain breakage, thus achieving the reconnection and growth of molecular chains. The molecular weight, intrinsic viscosity, or melt flow rate of the recycled material can be restored to over 90% of the virgin material level.

[0020] This invention integrates the two core processes of extraction and reconstruction into the same reaction system. The supercritical fluid serves both as the extraction medium to remove impurities and as the reconstruction reaction atmosphere to promote molecular chain repair, achieving a dual-purpose medium. Compared with stepwise processes, the process is shortened by 30-40%, energy consumption is reduced by more than 25%, and secondary pollution during material transfer is avoided. Attached Figure Description

[0021] Figure 1 This is a flowchart illustrating the present invention. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0023] The waste plastic recycling and processing method of the present invention, such as Figure 1 As shown, it includes the following steps: S1. Pre-treatment: sorting, crushing and washing waste plastics.

[0024] Different plastics have different properties, so they are first sorted to separate different types of plastics and group the same type together. After sorting, the plastics are crushed into particles with a diameter of 5-20mm. Then the plastic particles are washed to remove dust, oil, and other impurities, and dried so that the moisture content of the plastic particles does not exceed 1%.

[0025] S2. Low-temperature critical extraction: The pretreated plastic is sent into a high-pressure reactor, and an extraction solvent is injected into the high-pressure reactor. Extraction is carried out at a temperature 50-100°C lower than the melting point of the plastic and at a pressure higher than the critical pressure of the extraction solvent to separate and remove additives and small molecule impurities.

[0026] Specifically, the extraction solvent includes a main solvent and a co-solvent. The main solvent is carbon dioxide, propane, or dimethyl ether, preferably carbon dioxide. The co-solvent is one or more of ethanol, acetone, and ethyl acetate. During extraction, the main solvent is first liquefied under pressure at room temperature. Then, the liquid main solvent and liquid co-solvent are introduced into a mixer. After mixing, the mixture is introduced into a high-pressure reactor. In the mixer, the volume of the main solvent (in its liquid state) accounts for 60-90%.

[0027] The extraction temperature varies for different types of plastics. Specifically, the extraction temperature for conventional plastics such as polyolefins (PE / PP), polyesters (PET / PBT), and polystyrene (PS / ABS) is determined by their own melting point, and can be as low as 50-100°C below the melting point.

[0028] Regardless of the type of plastic, the extraction pressure is 8-25 MPa, preferably 12-20 MPa; the extraction time is 30-120 min, preferably 45-90 min; the extraction solvent is introduced into the high-pressure reactor at a rate of 0.5-5 L / min, using continuous dynamic extraction, and the extraction solvent that has passed through the plastic particles is also discharged from the reactor at a rate of 0.5-5 L / min. The total amount of extraction solvent introduced to the weight ratio of plastic is (3-10):1, preferably (5-8):1.

[0029] Under these temperature (50-100℃ below the melting point of plastics) and pressure (above the critical pressure of the extraction solvent system), the extraction solvent is in a subcritical or supercritical state, possessing both the high permeability of gases and the high solubility of liquids. Solvent molecules can penetrate into the polymer matrix, causing the polymer matrix to swell. Plasticizers, flame retardants, small molecules of color masterbatches, unreacted monomers, and odor molecules in the plastic are selectively dissolved due to differences in solubility parameters and carried out of the matrix with the flow of solvent molecules, resulting in a high removal rate. This method solves the problems of strong odor, small molecule precipitation, and unstable performance of recycled materials in traditional recycling in one step.

[0030] The extraction solvent discharged from the high-pressure reactor is fed into a distillation column with 15-20 trays and a reflux ratio of 2-5. The distillation column separates the additives and small molecule impurities in the extract phase to obtain an extraction solvent that can be reused.

[0031] S3. In-situ reconstruction: After extraction, a reconstruction agent is added to the high-pressure reactor, and the molecular chain repair reaction is carried out in a supercritical fluid atmosphere.

[0032] In this step, without removing the material, the reaction atmosphere is switched or adjusted in the reactor to chemically reconstruct the purified but chain-broken plastic matrix after extraction. This step consists of two stages: the first stage is the uniform mixing of the reconstructing agent and the plastic, and the second stage is molecular chain repair.

[0033] The supercritical fluid atmosphere can be achieved by the following operation: after extraction, by maintaining the system pressure above the critical point and switching to pure carbon dioxide feed, a supercritical carbon dioxide atmosphere is formed in the reactor. Then, the feed and discharge channels of the reactor are closed, and the in-situ reconstruction of step S3 is performed.

[0034] Reconstruction Principle: For condensation polymers (PET / PC), the epoxy groups in the reconstructing agent react with the hydroxyl and carboxyl groups generated at the ends of the polymer chains, achieving molecular chain reconnection through transesterification or urethane reaction. The intrinsic viscosity can be restored from 0.42 dL / g to 0.75–0.90 dL / g, approaching or reaching the level of virgin raw materials. For addition polymers (PE / PP), an organic peroxide initiator (such as dicumyl peroxide) at 0.1–0.5% of the plastic mass is added to the reactor simultaneously with or before the addition of the reconstructing agent to activate the free radical reaction of the reconstructing agent. Under the action of the initiator, the reconstructing agent generates free radicals, which react with the double bonds or active end groups formed by chain scission, achieving chain growth and branching.

[0035] Traditional reconstructing agents are typically epoxy chain extenders (ADR series), diisocyanates (MDI), maleic anhydride grafts, etc., which repair broken molecular chains through covalent bonding, effectively improving the molecular weight and mechanical properties of recycled plastics. However, these traditional reconstructing agents have the following inherent drawbacks: once the covalent bonds are formed, they are fixed, and the reconstructed plastic cannot undergo secondary repair or adaptive adjustment when subjected to heat or stress again; traditional small molecule chain extenders have limited compatibility with the plastic matrix, which can easily lead to uneven local reactions or precipitation; even after extraction treatment, waste plastics may still have trace impurities remaining, affecting the reaction efficiency of traditional chain extenders.

[0036] The reconstructing agent of this invention is a four-armed star copolymer having a triazine-1,3,5-tribenzamide supramolecular core, a polycaprolactone flexible linker, and glycidyl methacrylate-butyl methacrylate copolymer side chains, with the following general structural formula: ,

[0037] ST is a triazine-1,3,5-tribenzamide supramolecular unit, PCL is polycaprolactone with a molecular weight of 2000-5000, and GMA-co-BMA is a random copolymer side chain of glycidyl methacrylate and butyl methacrylate with a degree of polymerization m=10-30. The molar ratio of GMA to BMA is 3:1 to 1:1. The weight-average molecular weight of the reconstructing agent is 3.0×10⁻⁶. 4 -6.0×10 4 The molecular weight distribution is ≤1.5, and the epoxy equivalent is 350-450 g / eq.

[0038] When using the reconstructing agent of this invention, the reconstruction uses carbon dioxide as a supercritical fluid, and the specific reconstruction process includes: S31. After extraction, maintain a supercritical carbon dioxide atmosphere in the reactor and add the reconstructing agent to the high-pressure reactor. The amount of reconstructing agent added is 1.0-5.0% of the plastic mass. S32. Stir and mix at 150-180℃ for 5-10 minutes to ensure uniform dispersion of the reconstructing agent.

[0039] At temperatures of 150-180℃, the reconstructing agent molecules are in a molten state. The flexible PCL segments (molecular weight 2000-5000) swell significantly in supercritical carbon dioxide, granting the reconstructing agent molecules a high degree of freedom of movement. The four-armed star structure unfolds, with the GMA-co-BMA side chains extending outwards. The plasticizing effect of supercritical carbon dioxide increases the inter-chain spacing on the surface of the plastic matrix, forming nanoscale channels along which the reconstructing agent molecules diffuse into the interior of the plastic matrix.

[0040] During the above process, the hydrogen bonds between ST supramolecular units partially dissociate at this temperature to avoid premature aggregation. The epoxy groups of the GMA side chains remain unreacted, reserving activity for subsequent chain extension. The reconstructing agent molecules physically entangle with the plastic matrix through PCL segments, achieving uniform dispersion.

[0041] S33. Slowly reduce the temperature of the high-pressure reactor (e.g., 5℃ / min) to 80-100℃ and hold for 10-20 minutes to promote supramolecular self-assembly. During this stage, there should be no stirring or very slow stirring to keep the system in a quasi-static state.

[0042] As the temperature decreases, the thermal motion of molecules weakens, and the ST supramolecular units begin to self-assemble in an orderly manner through triple hydrogen bonds. The triazine-1,3,5-tribenzamide structure of each ST unit can form three hydrogen bonds, which recognize and pair with adjacent ST units, gradually forming nanoscale supramolecular aggregates.

[0043] In summary, supramolecular networks exhibit temperature reversibility: they form at a certain temperature and dissociate reversibly when heated above 120°C; supramolecular networks serve as templates, providing spatial guidance for subsequent covalent reactions.

[0044] S34. Heat to 200-240℃ and react for 15-30 minutes to achieve covalent chain extension. This stage is carried out under closed static conditions. The pressure may increase slightly with increasing temperature, but it remains in the supercritical state.

[0045] When the temperature is raised above 200℃, the hydrogen bonds in the ST supramolecular network reversibly dissociate, and the supramolecular aggregate structure softens and loosens. However, the reconstructing agent has already achieved uniform dispersion through low-temperature self-assembly, avoiding aggregation at high temperatures and laying a good dispersion foundation for subsequent efficient covalent chain extension. The epoxy groups on the GMA side chains are activated at high temperatures and begin to react with the active groups generated at the ends of the plastic chain scissions. For condensation plastics (PET, PC, PA): the epoxy groups undergo ring-opening reactions with the hydroxyl (-OH) and carboxyl (-COOH) groups at the chain-severing ends. The reaction mechanism is as follows: the epoxy groups open under the attack of nucleophiles (-OH, -COOH), forming ester or ether bonds, thus reconnecting the broken molecular chains. Each reconstitution agent molecule contains 40-120 epoxy groups, which can simultaneously connect to multiple chain-severing sites, achieving multi-point chain extension.

[0046] For addition polymers (PE, PP, PS): In the presence of an initiator (such as peroxide), the remodeling agent can generate free radicals, which can undergo addition reactions with the double bonds or unsaturated end groups formed by chain scission, achieving chain growth and branching. Tertiary C-H atoms in the side chains of GMA-co-BMA readily form free radicals under the action of an initiator and participate in the reaction.

[0047] The supramolecular assembly structure formed in step S33 plays a pre-dispersion and guiding role at this stage. Although the hydrogen bonds between ST units undergo reversible dissociation after heating, the low-temperature self-assembly in the early stage has enabled the reconstructing agent to achieve uniform distribution and locally ordered arrangement in the plastic matrix, effectively avoiding problems such as agglomeration and segregation of the reconstructing agent at high temperatures. Based on this uniform dispersion, the epoxy groups of the GMA side chain can more fully contact the active sites at the ends of the plastic chain scissions and undergo chain extension reactions, which significantly improves the chain extension efficiency, narrows the molecular weight distribution of the product, and makes the overall performance of the recycled plastic more stable.

[0048] Step S34 requires high-temperature activation of the chain extension reaction, during which plastic melting is normal. The key to this invention is maintaining a solid state during the extraction stage to avoid thermo-oxidative aging, thus providing a high-quality molecular chain foundation for reconstruction. Although the high temperature during the reconstruction stage causes the plastic to melt, the broken chain end groups have been repaired by the reconstruction agent, preventing performance degradation.

[0049] In summary, the reconstructing agent molecules initially exist in a monomolecular dispersion state, with each molecule consisting of a central ST-PCL-ST core and four GMA-co-BMA side chains. There are no hydrogen bonds between ST units, and the epoxy groups are in an unreacted state. The ST units are interconnected by triple hydrogen bonds, forming a two-dimensional hydrogen-bonded network. Multiple two-dimensional networks stack to form a three-dimensional supramolecular framework. The reconstructing agent molecules are interconnected through this supramolecular network, but there are no covalent bonds between molecules. The GMA epoxy groups react with the broken chain end groups of the plastic to form a covalently cross-linked network. Simultaneously, upon cooling, the ST units reform hydrogen bonds, and the supramolecular network recovers. Ultimately, a dual-network structure of interpenetrating covalent and supramolecular networks is formed: the covalent network provides permanent molecular weight enhancement and mechanical strength; the supramolecular network provides reversible toughening and a certain degree of self-healing function; the two networks are interconnected through the physical entanglement of PCL segments and the hydrogen bonds of the ST units, forming a unified whole.

[0050] After employing the reconstructing agent and method of this invention, the molecular weight of the plastic is significantly restored and enhanced thanks to the spatial accessibility of the PCL flexible chains and the directional reaction guided by the supramolecular template. The covalent network provides basic strength, and the supramolecular network absorbs energy through reversible hydrogen bond breakage and recombination under stress, avoiding stress concentration that could lead to brittle fracture, thus enhancing the mechanical properties of the plastic. This is due to the reversibility of the triple hydrogen bonds in the ST supramolecular units: upon heating, hydrogen bonds dissociate, the molecular chains gain mobility, and the ST units at both ends of the crack approach each other; upon cooling, hydrogen bonds recombine, the supramolecular network recovers, and the crack heals. The covalent network acts as a skeletal support during the repair process, preventing excessive material flow and deformation. The reconstructed recycled plastic exhibits a certain degree of thermotropic reversible self-healing capability.

[0051] The preparation method of the reconstructing agent of the present invention is as follows: Synthesis of the supramolecular core ST of triazine-1,3,5-tribenzamide; ST-PCL-ST prepolymer was synthesized by esterification reaction of ST with dihydroxyl-terminated polycaprolactone. ST-PCL-ST was reacted with 2-bromoisobutyryl bromide to introduce ATRP initiation sites, thus obtaining a macromolecular initiator; A reconstructing agent was obtained by grafting the side chains of glycidyl methacrylate and butyl methacrylate copolymers onto a macromolecular initiator via atom transfer radical polymerization.

[0052] A specific and feasible method for preparing the reconstructing agent is as follows: Synthesis of supramolecular core ST: Cyanurium chloride (1.0 mol) was dissolved in anhydrous tetrahydrofuran (2 L), and a tetrahydrofuran solution of p-aminobenzoic acid (3.3 mol) and triethylamine (3.3 mol) was slowly added dropwise over 2 h at 0-5 °C. The mixture was heated to 60 °C and reacted for 6 h, then cooled to room temperature, filtered, and the filter cake was washed three times with ethanol and dried under vacuum at 60 °C for 24 h to obtain a white solid ST with a yield of 92%.

[0053] Synthesis of ST-PCL-ST prepolymer: ST (0.1 mol) was dissolved in N,N-dimethylformamide (500 mL), and dicyclohexylcarbodiimide (0.22 mol) and 4-dimethylaminopyridine (0.02 mol) were added. The mixture was stirred for 30 min under nitrogen protection. Bis-hydroxyl-terminated polycaprolactone (PCL-diol, molecular weight 3000, 0.05 mol) was added, and the reaction was carried out at room temperature for 24 h. After the reaction was complete, dicyclohexylurea was removed by filtration. The filtrate was precipitated with diethyl ether, filtered, and dried under vacuum at 40 °C for 24 h to obtain a pale yellow solid ST-PCL-ST, with a yield of 85%.

[0054] Synthesis of ATRP macromolecular initiator: ST-PCL-ST (0.01 mol) was dissolved in anhydrous dichloromethane (200 mL), and triethylamine (0.05 mol) was added. A solution of 2-bromoisobutyryl bromide (0.05 mol) in dichloromethane was slowly added dropwise over an ice bath for 1 h. The reaction was allowed to proceed at room temperature for 12 h. The reaction solution was washed three times with saturated sodium bicarbonate, dried over anhydrous sodium sulfate, filtered, concentrated by rotary evaporation, precipitated with diethyl ether, and dried under vacuum at 40 °C for 24 h to obtain the ATRP macromolecular initiator in 78% yield.

[0055] Graft polymerization of GMA-co-BMA side chains: The obtained ATRP macromolecular initiator (0.001 mol), glycidyl methacrylate (GMA, 0.3 mol), butyl methacrylate (BMA, 0.1 mol), cuprous bromide (0.005 mol), and pentamethyldiethylenetriamine (0.01 mol) were added to a reactor. The reactor was purged with nitrogen three times, and anhydrous toluene (200 mL) was added. The temperature was raised to 80 °C and the reaction was carried out for 12 h. After the reaction, the catalyst was removed by passing it through a neutral alumina column. The filtrate was concentrated, precipitated with methanol, and dried under vacuum at 40 °C for 24 h to obtain a four-arm star copolymer with ST-PCL-ST as the central chain, which served as the reconstructing agent, with a yield of 72%.

[0056] S4. Extrude and granulate the reconstructed plastic melt.

[0057] During cleaning, an alkaline solution can be used first, followed by a rinse with clean water. To improve subsequent extraction and reconstruction effects, the preferred embodiment of this invention is as follows: During cleaning, the broken plastic is immersed in a cleaning solution containing cationic surfactants and cleaned at 60-75℃ for 20-40 minutes, controlling the residual cationic surfactant content on the plastic surface to be 0.01-0.1 wt% after cleaning. If the residual cationic surfactant content is below 0.01 wt%, the synergistic effect is insufficient; if it is above 0.1 wt%, it may interfere with the reconstruction reaction or lead to excessive surfactant content in the final product. By controlling the concentration, temperature, and time of the cleaning solution, the residual content can be precisely controlled within the above range.

[0058] The cationic surfactant is hexadecyltrimethylammonium bromide (CTAB) or a quaternary ammonium salt surfactant with C12-C18 alkyl chains. The cleaning solution comprises, by weight, 0.5-2.0 parts hexadecyltrimethylammonium bromide, 0.3-1.5 parts nonionic surfactant, 1.0-3.0 parts bio-based solvent, 0.1-0.3 parts complexing agent, and 93.2-98.1 parts water. The nonionic surfactant is an ethylene oxide-propylene oxide block copolymer, the bio-based solvent is limonene or terpineol, and the complexing agent is EDTA (ethylenediaminetetraacetic acid).

[0059] Cleaning solutions containing cationic surfactants can not only improve the cleaning effect, but also enhance the subsequent extraction and reconstitution effects.

[0060] In step S2, the process is carried out under a supercritical carbon dioxide atmosphere, meaning that carbon dioxide is used as the main solvent. In supercritical carbon dioxide, the residual cationic surfactant, together with a small amount of co-solvent (such as ethanol) in the extraction solvent or trace amounts of water in the system, forms a reverse micelle structure. Its polar core encapsulates the extracted polar additive, preventing it from redepositing on the plastic surface.

[0061] In step S3, the quaternary ammonium cations of the cationic surfactant remaining on the plastic surface form a cation-π interaction with the triazine ring of the ST unit. At the same time, the C16 long chain of the cationic surfactant becomes entangled with the PCL chain segment, which together promotes the orientation self-assembly of the reconstructing agent.

[0062] Specifically, cationic surfactants play a crucial role in this temperature range: the quaternary ammonium cations of the cationic surfactant form ion-π interactions with the triazine rings of the ST units, while the C16 long chains of the cationic surfactant become entangled with the PCL segments. At 80-100℃, the molecular chains of the cationic surfactant tend to arrange themselves in an ordered manner. This ordered arrangement induces the ST units to orient themselves in a specific direction through ion-π interactions, transforming supramolecular self-assembly from random aggregation to orientation-induced assembly. The ST units form a two-dimensional hydrogen bond network through triple hydrogen bonds, and multiple two-dimensional networks stack to form a three-dimensional supramolecular framework. These supramolecular frameworks use the cationic surfactant as nucleation sites, forming uniformly distributed physical crosslinking points in the plastic matrix, spaced approximately 50-100 nanometers apart. It is evident that the presence of cationic surfactants increases the orderliness and uniformity of the supramolecular network.

[0063] Synergistic effect with cationic surfactants: Ion-π interactions induce the orientational self-assembly of ST units, improving the order of supramolecular networks; cationic surfactants act as heterogeneous nucleation sites, reducing the critical concentration for ST self-assembly and accelerating self-assembly kinetics; the bridging structure of cationic surfactants enhances the interfacial bonding between the reconstructing agent and the plastic matrix, thereby improving the intrinsic viscosity and tensile strength of the plastic.

[0064] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for recycling and processing waste plastics, characterized in that, Includes the following steps: S1. Pre-treatment: sorting, crushing and washing waste plastics; S2. Low-temperature critical extraction: The pretreated plastic is sent into a high-pressure reactor and an extraction solvent is injected into the high-pressure reactor. Extraction is carried out at a temperature 50-100°C lower than the melting point of the plastic and at a pressure higher than the critical pressure of the extraction solvent to separate and remove additives and small molecule impurities. S3, In-situ Reconstruction: After extraction, a reconstruction agent is added to the high-pressure reactor, and a molecular chain repair reaction is carried out under a supercritical fluid atmosphere; S4. Extrude and granulate the reconstructed plastic melt.

2. The waste plastic recycling method as described in claim 1, characterized in that, In step S2, the extraction solvent includes a main solvent and a co-solvent. The main solvent is carbon dioxide, propane or dimethyl ether, and the co-solvent is ethanol, acetone or ethyl acetate. The main solvent is first liquefied under pressure, and then the main solvent and co-solvent are fed into a mixer. After mixing, the mixture is fed into a high-pressure reactor. In the mixer, the volume ratio of the main solvent is 60-90%.

3. The waste plastic recycling method as described in claim 1, characterized in that, In step S2, the extraction pressure is 8-25 MPa, the extraction time is 30-120 min, the extraction solvent is introduced into the high-pressure reactor at a rate of 0.5-5 L / min, and the weight ratio of extraction solvent to plastic is (3-10):

1.

4. The waste plastic recycling method as described in claim 1, characterized in that, In step S2, the extraction solvent discharged from the high-pressure reactor is fed into a distillation column to separate the additives and small molecule impurities in the extraction phase, thereby obtaining an extraction solvent that can be reused.

5. The waste plastic recycling method as described in claim 1, characterized in that, In step S3, the reconstructing agent is a four-armed star copolymer with a triazine-1,3,5-tribenzamide supramolecular core, polycaprolactone flexible linker, and glycidyl methacrylate-butyl methacrylate copolymer side chains, and its general structural formula is: ， Among them, ST is the supramolecular unit of triazine-1,3,5-tribenzamide, PCL is polycaprolactone with a molecular weight of 2000-5000, GMA-co-BMA is the side chain of a random copolymer of glycidyl methacrylate and butyl methacrylate with a degree of polymerization m=10-30, and the molar ratio of GMA to BMA is 3:1 to 1:

1.

6. The waste plastic recycling method as described in claim 5, characterized in that, The weight-average molecular weight of the reconstructing agent is 3.0 × 10⁻⁶. 4 -6.0×10 4 The molecular weight distribution is ≤1.5, and the epoxy equivalent is 350-450 g / eq.

7. The waste plastic recycling method as described in claim 5, characterized in that, The preparation method of the remodeling agent is as follows: Synthesis of the supramolecular core ST of triazine-1,3,5-tribenzamide; ST-PCL-ST prepolymer was synthesized by esterification reaction of ST with dihydroxyl-terminated polycaprolactone. ST-PCL-ST was reacted with 2-bromoisobutyryl bromide to introduce ATRP initiation sites, thus obtaining a macromolecular initiator; A reconstructing agent was obtained by grafting the side chains of glycidyl methacrylate and butyl methacrylate copolymers onto a macromolecular initiator via atom transfer radical polymerization.

8. The waste plastic recycling method as described in claim 5, characterized in that, Step S3 is carried out in a carbon dioxide atmosphere and specifically includes: S31. Add the reconstructing agent to the high-pressure reactor. The amount of reconstructing agent added is 1.0-5.0% of the plastic mass. S32. Mix at 150-180℃ for 5-10 minutes to ensure uniform dispersion of the reconstructing agent; S33. Cool to 80-100℃ and hold for 10-20 minutes to promote supramolecular self-assembly; S34. Heat to 200-240℃ and react for 15-30 minutes to achieve covalent chain extension.

9. The waste plastic recycling method as described in claim 5, characterized in that, In step S1, during cleaning, the broken plastic is immersed in a cleaning solution containing a cationic surfactant and cleaned at 60-75°C for 20-40 minutes, controlling the residual amount of cationic surfactant on the plastic surface to be 0.01-0.1 wt% after cleaning; the cationic surfactant is hexadecyltrimethylammonium bromide or a quaternary ammonium salt surfactant with C12-C18 alkyl chains. In step S2, the process is carried out under a supercritical carbon dioxide atmosphere. The cationic surfactants remaining on the plastic surface form reverse micelles, which encapsulate the extracted additives and prevent them from redepositing on the plastic surface. In step S3, the quaternary ammonium cations of the cationic surfactant remaining on the plastic surface form a cation-π interaction with the triazine ring of the ST unit. At the same time, the C16 long chain of the cationic surfactant becomes entangled with the PCL chain segment, which together promotes the orientation self-assembly of the reconstructing agent.

10. The waste plastic recycling method as described in claim 9, characterized in that, The cleaning solution comprises, by weight, 0.5-2.0 parts of hexadecyltrimethylammonium bromide, 0.3-1.5 parts of nonionic surfactant, 1.0-3.0 parts of bio-based solvent, 0.1-0.3 parts of complexing agent, and 93.2-98.1 parts of water.

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